Acoustic pulse echo techniques for investigating a casing set in a borehole are known in the art. In one technique as shown in U.S. Pat. No. 4,255,789 to Havira, a plurality of acoustic transducers are mounted around a tool to each direct acoustic energy at the wall of a borehole and detect acoustic reflections. The transducers are radially mounted to transmit and receive energy in a radial direction. Although this technique is particularly effective for evaluating certain characteristics such as the cement bond behind the casing or its thickness, the investigation with such spaced transducers does not yield information on the entire wall. A technique could be employed to use a mechanical scanning arrangement as shown in this patent. However, mechanical rotational components are preferably avoided in a hostile environment such as a mud-filled borehole.
Another pulse-echo technique for testing the wall thickness of a casing is described in U.S. Pat. Nos. 4,022,055 and 4,412,315. These describe a plurality of acoustic transducers that are arranged in a circle around the axis of a tool. The transducers are oriented to direct the acoustic pulses in an axial direction, i.e., a direction that is parallel to the axis of the tool. Acoustic energy is incident upon a conical reflecting surface which reflects each pulse towards the casing. The reflecting surface is split into two separate conical surfaces that are axially off-set from each other. The conical reflecting surfaces may be concave, convex or straight. A split reflection surface as described is not considered by this inventor to be useful for investigating a borehole casing with an acoustic pulse echo technique.
Ultrasonic transducers have been employed in an array. For example in the U.S. patents to Green, U.S. Pat. No. 3,177,382 and to Holloway U.S. Pat. No. 3,845,333 segments of transducers are shown arranged to radiate acoustic energy in radial directions. A linear array of transducers is shown in U.S. Pat. No. 4,425,525 and a planar array is shown in U.S. Pat. No. 4,344,159. A curved array of transducers arranged on an arc of a circle is described in U.S. Pat. No. 4,281,550. The latter array radiates and receives acoustic energy that travels in the direction of the center of the arc. A planar array of transducer segments is shown in U.S. Pat. No. 4,307,613 with groups of segments being so excited as to approximate the shape of a particular element. An array of transducers with selective phase shifting is described in U.S. Pat. No. 4,271,490. A technique for ultrasonic testing for flaws by focusing acoustic energy from successive transmitter groups is described in U.S. Pat. No. 3,693,415.
Techniques have been described for acoustically determining the thickness of an object by determining the thickness resonance frequency of the object. One technique as described in U.S. Pat. Nos. 3,741,334 and 3,914,987 involves directing an acoustic pulse at the object with a pulse whose frequency is initially set at the normal thickness resonance frequency of the object. The acoustic reverberations in the object are then detected and amplified for use in making another measurement of the thickness resonance. This procedure may be repeated until the frequency of reverberations accurately represent the thickness of the object.
It is desirable to detect defective areas in a casing that is set in a borehole to identify corroded areas to thus be able to take corrective steps to avoid blow-outs and contamination of and by water bearing zones. The casing thicknesses indicative of such defects may vary from a nominal value and it is, therefore, desirable to measure the casing thickness in an accurate manner.